524 U. S. BUREAU OF FISHERIES 



to the combination of erosion silt and organic waste which the erosion 

 silt carries along from the sewerage and wastes introduced into the 

 river by cities and manufacturing plants. Laboratory tests carried 

 forward under controlled conditions show the decomposition of 

 various organic substances mixed with erosion silt to proceed at a 

 slower rate and to produce different end products with greater oxy- 

 gen demands than the decomposition of these same substances in 

 plain water alone. 



Cotiiparison of silts. — Chemical studies and electrical and physical 

 measurements on erosion silts from various parts of the Mississippi 

 Valley have been made and technical classifications of these silts 

 obtained, which permit comparisons of the erosion silt action 

 through the Mississippi River system. 



Erosion silt and industrial pollution. — Detailed laboratory studies 

 on the relation of erosion silt to industrial pollution, particularly 

 sulphur, acid wastes, and heavy metal wastes, are in progress, con- 

 tinuing the work on sulphur, acid pollution, and arsenic begun last 

 year. These studies so far indicate that the erosion of silt is both 

 a carrier and reservoir for many of these substances, and the pres- 

 ence of erosion silt in many cases augments the detrimental action of 

 these substances on living organisms. 



MUSSEL INVESTIGATIONS 



The overwhelming of so many mussel beds by the silt deposits is 

 so evident in the Mississippi and Tennessee systems that in view of 

 the proposed modification of many of the inland streams for naviga- 

 tion and hydroelectric plants careful attention has been given to the 

 condition of the existing mussel beds wherever found throughout the 

 field work, and extensive collections of all species, both commercfal 

 and noncommercial, were made for survival and population studies. 

 Upward of 10,000 shells have been examined in the last year and 

 detailed statistical studies made of the age, rate of growth, weight, 

 thickness, and of the physiological condition of the animal itself at 

 the time of capture for comparison with the physical and chemical 

 data obtained in the field at the habitat from which the individual 

 mussel was taken. In this way it has been found that the natural 

 replacement at present of fresh-water mussels of the commercial 

 species in the Upper Mississippi River is far below that required for 

 even the maintenance of the existing beds. The statement is also 

 true to a large extent for the portions of the Ohio and Tennessee 

 systems studied. These facts have been set forth in the preliminary 

 report (Fisheries Circular No. 7, 1931). With the depletion of the 

 mussel beds of these major streams almost a certainty if existing 

 conditions continue, detailed physical, chemical, and biological 

 studies have been made in the most favorable haTiitats found with 

 a view to a duplication of such conditions in controlled waters. 

 From the data obtained experimental raceways have been planned 

 and experiments will be started this spring in the Lake Worth area 

 with a view to duplicating the best conditions for mussel growth and 

 the rearing of large numbers of mussels in small areas. 



The transfer of attention from the reestablishment of the mussel 

 beds in the natural habitats in the larger rivers, as the Mississippi 



